This project is defining a potential animal model of mucosal epithelial proliferation of the colon. TMCH is a naturally occurring infectious disease of mice characterized by colonic mucosal hyperplasia. It is caused by a specific variant of Citrobacter freundii. Sequential studies are correlating findings of gross pathology, light microscopy, bacteriology and electron microscopy during the course of the disease. Cell kinetic autoradiography techniques are confirming the prolonged cell cycle time, prolonged S phase, and surface mucosal mitotic activity which resembles neoplasia. The mechanisms of lesion regression are being analysed using a number of approaches including immunology, in an effort to determine if TMCH can be manipulated to progress to neoplasia. Additional efforts include examining the interaction of TMCH with chemical carcinogen, 1,2-dimethylhydrazine (DMH). TMCH reduces the latent period of DMH carcinogenesis. Diet plays a role in TMCH, and the effect of diet on lesion genesis is being studied.